Bone densitometer

ABSTRACT

In an x-ray bone densitometer, special calibration techniques are employed to accommodate variations. In one aspect, a bone-like calibration material is interposed and the system determines the calibration data from rays passing only through flesh. In another aspect, a rotating device carries the calibration material through the beam. The specific densitometer shown uses an x-ray tube operated at two different voltages to generate a pencil beam, the energy levels of the x-ray photons being a function of the voltage applied. An integrating detector is timed to integrate the detected signal of the patient-attenuated beam over each pulse, the signals are converted to digital values and a digital computer converts the set of values produced by the raster scan into a representation of the bone density of the patient. Multiple reference detectors with differing absorbers are used by the system to continuously correct for variation in voltage and current of the x-ray tube. Calibration is accomplished by the digital computer on the basis of passing the pencil beam through known bone-representing substance as the densitometer scans portions of the patient having bone and adjacent portions having only flesh. A set of detected signals affected by the calibration substance in regions having only flesh is compared by the computer with a set of detected signals unaffected by the calibration material.

The invention is an x-ray densitometer suitable to measure bone density,or density of bone like materials, in the human body, particularly inthe spine and hip. Such measurements are useful, e.g. for determiningwhether patients are affected by osteoporosis. The invention uses ameasurement technique which is an improvement over a related techniquecalled dual-photon absorptiometry. Dual photon absorptiometry is basedon the use of radioisotopic sources to provide photons of two differentenergies whereas the present invention uses an x-ray tube switchedbetween two different voltages in order to generate a collimated beam oftwo different energies.

An x-ray source is capable of producing an intensity of radiation about1000 times greater than conventional radioisotopic sources used for bonedensity measurements. If an x-ray source were successfully incorporatedin a bone densitometer, an improvement in measurement time, resolution,accuracy, precision, and minimization of radiation dose might beeffected. Prior efforts to use x-ray sources for bone densitometers havenot been altogether successful. A major purpose of the present inventionis to provide a successful bone densitometer and to achieve improvedperformance in all of the important categories by taking advantage ofthe high radiation intensity produced by an x-ray source.

The invention achieves this objective by use of an x-ray tube which ismoved in a 2-dimensional raster scanning pattern with a fixedrelationship between the tube and a collimator and detector which movewith it, with alternating high and low voltage levels being applied tothe x-ray tube.

In order to take optimal advantage of the high photon intensity providedby x-ray sources, the invention overcomes certain problems associatedwith using x-ray tubes for bone densitometry. Although x-ray sources aremore intense than radioisotopic sources, they are also less stablebecause they vary (drift) with changes in the voltage and currentsupplied to them. In addition, x-ray tubes produce photons that have abroad range of energies whereas radioisotopic sources typically producephotons with only a few energies.

These and other problems are met by a system which employs two referencedetectors instead of one, means for providing frequent bone calibration,e.g. on every scan line, preferably on every point, and use of anintegrating detector in the raster scan.

An important feature of the invention is a calibration technique whichdetermines the location of bone and then calibrates the system on thebasis of x-ray data produced from non bone areas that lie close to thelocation of bone. Such calibration not only accomodates drift of thex-ray source but also other variations encountered, such as variation inbody thickness from patient to patient.

To summarize, according to one aspect of the invention, a bonedensitometer is provided for measuring density of bone-like material ina patient who is held in fixed position, comprising an x-ray tube meanshaving a power supply, detector means arranged on the opposite side ofthe patient to detect x-rays attenuated by the patient, means toeffectively expose portions of the patient having bone and adjacentportions having only flesh (non-bone body substance), means for causingthe beam to pass through a bone like calibration material in the courseof the exposure, and signal processing means responsive to the output ofthe detector means to provide a representation of bone density of thepatient (e.g., an x-ray film like picture of the patient, showing bonedensity distribution or calculated values representing bone density ofthe patient), the signal processing means adapted to respond to databased upon x-rays attenuated by the calibration material in regionshaving only flesh to calibrate the output of the detector means, therebyenabling the accommodation of drift in the x-ray tube, differences inpatient thickness and other system variations.

According to another aspect of the invention, a bone densitometer, e.g.,having the features described above, further comprises a pencil beamcollimator arranged to form and direct a pencil beam of x-rays throughthe patient, and the detector means, on the opposite side of thepatient, is aligned with the collimator, the x-ray tubes, pencil beamcollimator and detector means adapted to be driven in unison in an X Yraster scan pattern relative to the patient, and rotating means causethe beam to pass through bone-like calibration means. According to thisaspect of the invention, the signal processing means is not limited toresponding to data based upon x-rays attenuated by the calibrationmaterial in regions having only flesh to calibrate the output of thedetector means.

According to another aspect of the invention, a bone densitometerincorporates features of both of the above described aspects of thisinvention.

In preferred embodiments of these aspects of the invention, the powersupply of the bone densitometer is adapted to apply alternate high andlow voltage levels to the x-ray tube; control means for the frequency ofthe voltage is related to the speed at which the x-ray tube, collimatorand detector means are driven in scan motion and the beam width producedby the collimator to apply alternating high and low voltage levels tothe x-ray tube at a frequency sufficiently high that at least one pairof high and low level exposures occurs during the short time periodduring which the pencil beam traverses a distance equal to about onebeam width, preferably the bone densitometer being adapted to producepairs of high and low voltage pulses at a rate of the order of sixty persecond, the x-ray tube, collimator and detector means being driven alongthe scan at a rate of the order of one inch per second and thecollimator produces a pencil beam of between about one and threemillimeters in diameter; the x-ray beam passes through the bone-likecalibration material at least once per scan line of a scan pattern for aperiod equal to at least the time during which one pixel of resolutionis traversed, preferably the x-ray beam passes through the bone-likecalibration material for the duration of every other high and lowvoltage pulse pair; the detector means comprises an integrating detectorcontrolled to integrate the detected signal repeatedly over short timeperiods relative-to the time required to advance the x-ray scan patternby one pixel of resolution, preferably an analog to digital converterbeing provided to convert each integrated value, to a digital signal anda digital computer means is provided for producing the representation ofbone density cf the patient by processing the stream of the digitalsignals; and a reference system is provided having at least tworeference detectors each provided with a different absorber, thereference system adapted to correct for both x-ray tube current andvoltage changes, preferably the system adapted to correct the detectedsignal substantially on the basis of a function of the signals producedby the reference detectors and, where there are two of the referencedetectors, the function being substantially a straight line defined bythe detected signals of the reference detectors.

The present invention makes it possible to perform bone densitymeasurements more rapidly and with better resolution and accuracy thanprior devices. Because it does not use radioisotopic sources, the userdoes not need to handle and replace radioactive materials which aredangerous and are strictly controlled by federal licensing regulations.

In the drawings:

FIG. 1 is a diagrammatic illustration of the preferred embodiment;

FIG. 2 represents a patient's spine with superposed scan pattern; FIG.2a illustrates the scan pattern employed by the preferred embodimentwhile FIG. 2b illustrates an alternative scan pattern;

FIG. 3 is a block diagram of the electronic control and measuring systemof the preferred embodiment;

FIG. 4 is a plan view of the calibration disc employed in the preferredembodiment;

FIG. 5 is an illustration of the voltage levels produced during the highenergy level (HEL) and low energy level (LEL) phases of energization ofthe x-ray tube as modified by the synchronized calibration wheel having"bone" and "no bone" quadrants;

FIG. 6 shows a plot of a function derived from the "bone" and "no bone"pulse pairs for a single traverse of the spine; and

FIGS. 7, 7a and 7b represent a flow diagram of the calculationsperformed by the computer for examination of a spine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the basic components of the x-ray densitometer. X-ray tube1 carried on x-y table arrangement 2 is energized by power supply 11which is designed to alternate its voltage output rapidly between twolevels called the "High Energy Level" (HEL) and the "Low Energy Level"(LEL). The HEL is typically 150 kilovolts and the LEL is typically 75kilovolts. The x-rays emitted by the x-ray tube are collimated to form apencil beam B by collimator 3. The pencil beam passes through acalibration disc 12 which rotates at a rate which is synchronized withthe rate at which the power supply 11 switches between the HEL and LEL.The role played by the calibration disc will be described below.

The x-ray pencil beam passes through, e.g., a female adult patient 4under examination for possible presence of osteoporosis and impinges ona main radiation detector 5. Two reference detectors 7 and 8 which aresimilar in design to the main detector 5 are also shown in FIG. 1. Thereference detectors 7 and 8 monitor the flux emitted by x-ray tube 1 andprovide information used to correct the signal measured by main detector5 for variations in the x-ray tube current and voltage.

The reference detectors 7 and 8 measure radiation from the x-ray tube 1after it has traversed one of two x-ray absorbers 9 and 10. The twoabsorbers 9 and 10 are of substantially different thicknesses which aretypically chosen to be representative of the x-ray attenuation of a thinpatient and a heavy patient respectively. By using two referencedetectors with different absorbers it is possible to monitor changessimultaneously in both the x-ray tube current and voltage. The referencedetector measurements are used to correct the measurements made with themain detector 5 in order to compensate for these changes in tube currentand voltage. The manner in which these corrections are made is describedbelow.

X ray tube 1, collimator 3, reference detectors 7 and 8 and absorbers 9and 10, calibration disc 11, and main detector 5 are all mechanicallycontinuously scanned in the X direction across the body during whichtime signals from the main detector 5 and reference detectors 7 and 8are digitized and stored in computer system 13. After each scan fromright to left or left to right in FIG. 1 the assembly briefly stopsmoving in the X direction, and is indexed a small amount in the Ydirection, out of the plane of FIG. 1. As a result of these motions, thepencil x-ray beam B undergoes a rectangular scanning pattern 18 such asshown in FIGS. 2 and 2a. A modified rectangular scanning pattern shownas parallelogram pattern 19 in FIG. 2b might also be used to measure abone such as the neck or the femur, which is set at an angle in thehuman body.

FIG. 1 illustrates the fixed relationship between the x-ray source 1 andmain detector 5 during the scanning period throughout which patient 4lies stationary on patient table 20. X ray tube 1, collimator 3,reference detectors 7, 8 and absorbers 9 and 10, and calibration disc 12are mounted together in a single assembly called the source assembly 22.This assembly is mounted in turn below the patient on a conventional X-Ytable 2. Separate stepping motors and lead screws are used to move the XY table in the X direction and Y-direction respectively. The steppingmotors and lead screws are of a type well known in the art and are notshown.

The main detector 5 is mounted above the patient and in the preferredembodiment shown is rigidly attached by means of C arm 21 to the sourceassembly 22 so that x-ray pencil beam B and main detector 5 have a fixedrelationship throughout the scan. The main detector 5, in alternativeembodiments, could be driven with its own drive system in either theX-direction, Y-direction or both so long as it maintains the same fixedrelationship to pencil beam B.

In FIG. 2, a representation of the patient's spine 6 and adjacentportions of the body is shown. In general, pencil beam B scans from sideto side across the patient's spine, and through flesh on either side ofthe spine, but does not pass beyond the outer dimensions of the adultpatient 4. The total distance scanned from side to side (i.e. in theX-direction) is typically 5 inches and the total distance scanned fromhead to toe (i.e. in the Y-direction) is typically 5 inches.

During the scanning period, the signals from detector 5 and fromreference detectors 7 and 8 are digitized and stored in computer system13. It is possible to calculate the bone density at each point in thescan pattern from these data using a method described in more detailbelow. Both the raw data and the calculated bone density can bedisplayed as an image using any one of a number of devices well known inthe art. Such an image will resemble a conventional x-ray image orradiograph. In a preferred embodiment, the computer system 13 containsone such device known as a display processor which displays the imageacquired in this manner on a television screen. Devices such as adisplay processor or other computer peripherals such as laser printersare well known devices for displaying images from digital data.

FIG. 3 is an electronic block diagram of the bone densitometer showingthe relationship between the different components of the system. X-rayphotons striking the crystals in the scintillation detectors 5, 7, 8generate optical radiation which is converted by the detectorphotomultiplier tubes into electrical currents. These in turn areamplified and converted to voltage levels by individual amplifiers 30.The amplifier outputs are integrated by respective integrators 32 fortime periods that are controlled by the system timing control 36 aboutwhich more will be said. The output of the three integrators aredigitized by an analog-to-digitial converter 34 and stored forprocessing in a small computer 13 such as an IBM PC or AT computersystem.

The system timing control 36 synchronizes the x-ray power supply pulsingand the signal integrators. For example, just before a HEL voltage isapplied to the x-ray tube, all three integrators are reset to zero. Asthe HEL is applied to the x-ray tube, radiation is emitted and all threeintegrators begin to integrate signals. A short time later (typically1/120 second), the system timing control terminates the HEL voltagelevel, terminating the emission of x-radiation. This is immediatelyfollowed by a signal generated by the timing control which terminatesthe integration of detector signals and causes the A/D converter todigitize the integrator output and transfer the digital value to thecomputer system. A similar sequence of timing signals is then generatedfor the next LEL pulse after which the cycle is repeated.

The system timing control 36, in addition to the functions describedabove, also provides a means to synchronize, via synchronizing circuit37, the calibration disc motor 12a to assure that the rotation frequencyof the calibration disc 12 and the x-ray pulsing frequency are lockedtogether so that the timing relationships illustrated in FIG. 5 aremaintained. The timing control in a preferred embodiment also provides apulse sequence to the stepping motor controller 38 which drives X andY-direction motors 2a and 2b and assures that every scan line in thex-ray image has exactly the same number and phasing of x-ray pulses.

The computer 13 provides scan distance instructions to the steppingmotor controller 38 and scan initiation instructions to the systemtiming control 36 and allows the operator to initiate, manipulate, andterminate the raster scan motion and x-ray generation by means of astandard keyboard. It records the digitized detector information,calculates bone density for each point in the raster scan pattern, anddisplays the resulting image using a standard computer display processorand television monitor. Hardcopy versions of the calculated anddisplayed bone density can be obtained with a standard printerinterfaced to the computer.

BEAM SIZE AND SWITCHING FREQUENCY

During the scanning of the x-ray pencil beam B, the voltage on the x-raytube 1 is switched between the HEL and LEL. A typical speed used tocontinuously scan across the patient from side to side is 1 inch persecond and a typical switching frequency for the x-ray tube power supplyis 60 cycles per second. In this case there will be 60 HEL pulsesalternating with 60 LEL pulses generated during each one second ofscanning. The signals from detectors 5, 7 and 8 are recorded separatelyfor the HEL pulses and for the LEL pulses. For a scan speed of 1 inchper second, one HEL/LEL pair of measurements is made for every 1/60 ofan inch (0.016 inch) traversed by the pencil beam.

The cross sectional area of the pencil beam B is determined by theopening in collimator 3 and is typically 1-3 mm (0.040-0.120 inch) sothat there are typically 21/2 to 8 HEL/LEL pulse pairs per beam width.One of the important features of the present invention is that there isat least about one pulse pair per beam width. As a result, the smallregion of the body sampled by the pencil beam during the HEL measurementwill be essentially the same as the small region of the body sampled bythe LEL measurement made 1/120 second later.

USE OF INTEGRATING DETECTORS

The detectors used in the preferred x-ray densitometer are of a typegenerally known as scintillation detectors, although use of a number ofother types of detectors is also possible. A scintillation detectorconsists of a crystal material coupled to a photomultiplier tube. Thecrystal serves to convert x-ray radiation to optical radiation and thephotomultiplier tube converts the optical radiation to an electronicsignal. Solid state photodiodes coupled to x-ray fluorescent screens,ionization chambers, and other devices might also serve as radiationdetectors for the present invention.

In dual-photon bone densitometers using radioisotopes, scintillationdetectors are also used as radiation detectors. However, in thesedevices, the detectors must detect individual x-ray photons and sortthese photons into two separate channels corresponding to high-energyphotons and low-energy photons. This requirement for performing aspectrum analysis on individually detected photons is dictated by thefact that the isotopic source emits both high-energy and low energyphotons simultaneously.

In the present invention, the scintillation detector used need notperform a spectrum analysis task by sorting photons into high andlow-energy channels because the high-energy and low-energy photons arenot emitted simultaneously. Rather the HEL and LEL voltages aregenerated alternating in time. The high-energy and low-energy photonsare integrated separately over the duration of the HEL and LEL pulsesrespectively and are therefore recorded at different times.

The ability to use energy integrating detectors rather than photoncounting detectors is an important feature of the x-ray densitometerbecause it makes it possible to complete a patient scan in a short time.In order to measure bone density to a given accuracy, it is necessary todetect a resulting minimum number of photons because the statisticalaccuracy of a measurement is related, as is well known, to the squareroot of the number of detected photons. For example, at least 50-100million photons are typically detected in a bone density measurement ofthe spine.

The x-ray densitometer of the present invention completes a measurementscan in as little as 2 to 5 minutes. In order to record as many as 100million photons in 2 minutes, the detector must record on the order of 1million photons per second. By using energy integrating detectors ratherthan pulse counting detectors, the x-ray densitometer can record photonfluxes of 1 million per second or higher. (An energy integratingdetector can easily record photon fluxes as high as 100 million photonsper second.) The use of alternating high and low voltage pulsing of thex-ray tube, coupled with integrating detectors to measure thehigh-energy and low-energy signals produced, is an important feature ofthe present invention because it makes short scan times possible, whichimplies a shorter visit by patients and better use of capital equipment.

THE REFERENCE DETECTORS AND ABSORBERS

By use of two reference detectors, with different absorbers, smallchanges in both the x-ray tube current and applied voltage areeffectively monitored along with the signals from the main detector.Just as the main detector integrates photons and supplies separatevalues for the HEL pulse and the LEL pulse, the reference detectors alsosupply separate reference values for each HEL pulse and LEL pulse. EachHEL or LEL measurement recorded by the main detector is correctedaccording to the following method.

Let P1 and P2 be the percentage changes in output signal (from a HEL orLEL pulse) measured by the first and second reference detectorsrespectively due to a small variation in x-ray tube current and voltage.Let T1 and T2 be the thicknesses of body tissue that attenuate the HELor LEL pulses the same amount as do the first and second absorbers shownin FIG. 1 respectively. Each main detector signal corresponds to a bodythickness T0, and is corrected by a percentage P0 which is given by theformula (P0-P1)=(P2-P1)/(T2-T1)*(T0-T1). (This can be recognized as theformula for a straight line fit between the measured values of P1 andP2.) This correction compensates the main detector measurement forchanges in both x-ray tube current and voltage. This feature enables themain detector signal to be corrected for fluctuations in x-ray tubecurrent and voltage that would otherwise degrade the accuracy of thebone density calculation, unless the x-ray power supply is quite stableover the duration of one patient scan. In the case that the power supplyis sufficiently stable, the use of the reference detectors may beomitted. Longer term variations in x-ray tube current and voltage arecompensated by the use of the calibration disc now to be described.

THE CALIBRATION DISC

FIG. 4 is a plan view of the calibration disc 12 which is mounted suchthat the region of the disc near the circumference interrupts pencilbeam B as the disc rotates. The calibration disc is synchronized to theswitching frequency of the high voltage power supply. In a preferredembodiment, the power supply produces HEL and LEL pulses which arederived from the main power line frequency of 60 Hertz. The HEL and LELpulses generated by the power supply in this embodiment are shown inFIG. 5. One pair of HEL and LEL pulses are generated every 1/60 of asecond.

In the preferred embodiment, the calibration disc is driven with asynchronous motor which rotates at a rate of exactly 30 revolutions persecond and which is adjusted in phase such that four pre-definedquadrants of the disc (labeled Q1, Q2, Q3, and Q4 in FIG. 4) correspondto the HEL and LEL levels being generated by the power supply. Morespecifically, when Quadrant 1 is obstructing the pencil beam the voltagelevel is HEL, when Quadrant 2 is obstructing the pencil beam the voltagelevel is LEL, when Quadrant 3 is obstructing the beam the voltage levelis again HEL, and finally when Quadrant 4 is obstructing the beam thevoltage level is again LEL. The desired synchronization between thequadrants of the calibration disc and the HEL/LEL voltage pulses isillustrated in FIG. 5.

In this preferred embodiment, the circumference of Quadrants 1 and 2consists of a material which has the same x-ray attenuationcharacteristics as bone. Both quadrants contain exactly the same amountof the bone-like calibration material 15 which typically amounts toabout 1 gram per square centimeter of material. As a result, every otherHEL/LEL pulse pair recorded by the main detector is attenuated by aconstant thickness of calibration bone, as the calibration bone rotatesin and out of the x-ray beam. Using the calibration disc in this manner,four distinct types of measurements are periodically recorded from themain detector. These types and their abbreviations are (1) HEL and nocalibration bone (H), (2) LEL and no calibration bone (L); (3) HEL withcalibration bone (HB), and (4) LEL with calibration bone (LB). The fourgroups of measurements H, L, HB, and LB conditions are illustrated inFIG. 5.

One additional function of the calibration disc is worth noting. It ispossible to make use of the same disc for the additional purpose ofproviding different x-ray filtration for the HEL and LEL x-ray beams.For example, in a preferred embodiment, the LEL beam is left unfilteredwhereas the HEL beam is filtered with 1 mm of copper. The purpose of thecopper filtration is to attenuate the HEL beam, which is typically ofconsiderably higher intensity than the LEL beam because it suffers lessattenuation in tissue. By attenuating the HEL beam, it is possible toavoid unnecessary x-ray exposure to the patient and thereby lower thedose without substantially affecting the accuracy of the final bonemeasurement.

In FIG. 4, in a preferred embodiment, Quadrants 1 and 3 contain thecopper filtration in the form of a constant thickness sheet of copper 20for the HEL beam and Quadrants 2 and 4 contain no filtration (orpossibly another filtration material) for the LEL beam. The use of arotating wheel to provide different x-ray filtrations is anotheradvantage of the present invention, although the main purpose of thecalibration disc is to provide continuous calibration of thedensitometer as explained below.

CALCULATION OF BONE DENSITY

The method used to calculate bone density with high accuracy is based ondual photon absorptiometry calculations as described in priorpublications. (See for example: "Noninvasive Bone Mineral Measurements,"by Heinz W. Wahner, William L. Dunn, and B. Lawrence Riggs, Seminars inNuclear Medicine, Vol. XIII, No. 3, 1983.) The x-ray densitometerdescribed here uses a modification of the established method which makesuse of the calibration disc bone-like material to obtain an absolutereference for making accurate and repeatable measurements of the realbone. The calibration disc measurements automatically and continuouslycalibrate the values calculated for bone density and thereby compensatefor any short or long term drift in the x-ray detection electronics orother system variations, as well as differences in patient thickness.

FIG. 6 shows schematically a plot of the function F=1n(L)-k*1n(H) forboth the "bone" and "no bone" pulse pairs for a single traverse acrossthe spine. It is important for this calibration to traverse across atleast some portions of the patient having only flesh adjacent to thespine. (Using the notation defined above to be more precise, thecalibration bone version of F, F(B) is equal to 1n(LB)-k*1n(HB) and theno calibration bone version of F, F(NB), is equal to 1n(L)-k*1n(H).) Inthese formulae, the symbol, 1n, indicates the natural logarithm functionand the letter, k, is equal to the ratio of the attenuation coefficientof tissue for the LEL pulse to the attenuation coefficient of tissue forthe HEL pulse. The values for H, L, HB, and LB used to calculate F inthese formulae are the x-ray beam attenuation values measured by themain detector after corrections derived from the reference detectormeasurements have been applied. The reference corrections applied inthis manner are given by the values for P0 as described above.

The Wahner et al. publication cited demonstrates that the value of thefunctions F(B) and F(NB) will be a constant if there is no bone orbone-like material (or other high atomic number material) in the beampath through the patient. This is strictly true if k is constant acrossthe scan line, independent of patient thickness, and can be made to holdin practice by measuring any dependence of k on patient thickness andusing the corrected value of k to calculate the function F. In FIG. 6,the resulting constant values obtained when the beam is on either sideof the spine (i.e. passing through portions of the body having flesh,without bone) are labeled Calibration Baseline and Normal Baselinecorresponding to the plots of the functions F(B) and F(NB) respectively.The increase in the value of the function, F, over and above eachbaseline level when the x-ray beam scans across the spine has been shownto be directly proportional to the amount of bone or bone-like materialin the path of the beam.

In FIG. 6, the separation value 16 between the Normal Baseline and theCalibration Baseline can be calculated by finding the numerical averageof the difference between F(B) and F(NB) for measurements made throughportions of the body having only flesh on either side of the bone. Thisseparation value is an important parameter in the operation of thedensitometer because it calibrates the bone measurements in the spinedirectly against the known density of the bone-like calibration materialwhich is used in the calibration disc measurements. Thus, the separationvalue corrects for both x-ray source drift, and for variations betweendifferent patients e.g. patient thickness, and other system variations.In FIG. 6, for example, suppose that a particular set of H/Lmeasurements results in an F value (17 in FIG. 6) equal to 2.46 at thespine, and that the average separation value 16 at adjacent portions ofthe body has a value of 1.23. Then the bone density in the spine at thepoint where F equals 2.46 will be exactly (in this example) 2.00 timesthe of the bone-like calibration material used in the calibration disc.

Using the separation value 16 as a calibration constant, the spine bonedensity can be calculated for all measurement pairs H/L recorded duringthe entire two dimensional raster scanning of the patient without theneed for precise mechanical positioning of the x-ray beam or thecalibration bone. The resulting two dimensional array of bone densityvalues can be displayed as an image using readily available computerperipherals.

Using the calibration disc to perform calibration measurements is thepresently preferred method of calibrating the x-ray densitometer.Because calibration measurements are made numerous times for every scanline in the bone density image, one obtains assurance of propercalibration regardless of the location of the bone in the scan pattern.Other means of performing calibration measurements are possible and arecomprehended by certain broader aspects of this invention. For example,if the pencil beam were brought to a rest at the end of each scan lineor at the start or completion of the entire san a piece of bone-likematerial could be inserted in the beam and calibration data could betaken. Alternatively, the bone-like calibration material could beinserted into the beam on every alternate scan line. In anotherembodiment, a small amount of calibration material is placed under apart of the patient having only flesh without bone.

After the baselines have been determined for each scan line and the bonedensity image has been created, it is also useful to detectautomatically the outer boundary of the spine for each scan line. Fromthese data it is possible to calculate a number of useful parameters.For example, the integral of the bone density values (expressed in unitsof grams per square centimeter) between two pre determined scan lines(usually chosen to correspond for spine measurements to a fixed numberof vertebral bodies) is called the total bone mineral content (expressedin grams) and is one often quoted parameter. The integral of bonedensity values over the entire region of interest divided by the area ofthe bone as determined by the outer boundaries of the spine is the areaaveraged bone mineral density (expressed in grams per square centimeter)and is another frequently cited parameter.

The means for determining baseline values for individual scan lines, themeans for calculating the boundary of the bone in the bone densityimage, and other details of the bone density calculation are well knownand are not novel to the present invention. The means for interceptingthe x-ray beam with a bone-like calibration material and calculating twobaselines at portions of the body having only flesh, to calibrate thesystem, has many advantages.

FIG. 7 is a flow diagram illustrating the steps performed by computersystem 13 to calculate bone mineral content of the spine while FIG. 7adiagrams the steps of the calibration routine, block 3 of FIG. 7 andFIG. 7b diagrams the routine for calculation of bone mineral content.

For calibration, step 3, the program examines the data and decides whichpoints in the scan form part of the spine and which do not. In otherwords, it distinguishes "spine" points from "flesh" points. It achievesthis very simply by setting respective thresholds for F(N) and F(NB)values and defines all values above the threshold as "spine" points andall values below the threshold as "flesh" points.

The program then subtracts the F(NB) points that overlay flesh from theF(B) points that overlay flesh to calculate the separation value S_(L)shown in FIG. 6 as distance 16. The average of the separation valuesaveraged over all such subtracted pairs of points is then used as acalibration constant for Step 15 of FIG. 7 to calibrate the bone mineralcontent, see FIG. 7b. If the separation value changes by two percent,for example, from one day to another because of x-ray tube drift thenthe uncalibrated bone mineral content will change by two percent but thecalibrated bone mineral content will remain constant.

Most of the steps shown in FIG. 7 are self-explanatory but someadditional explanation is helpful. After the separation value,calculated in Step 3, is subtracted from the F values for "bone", thereis no longer any distinction between the F values for "bone" and "nobone" and these two sets of F values may be merged into a single set inorder to increase the spatial resolution achieved by doubling the numberof points used to create the bone image (Steps 4 and 5).

In Step 7, the edges of the spinal vertebral bodies are found becauseFIG. 7 illustrates the flow sequence for measuring bone mineral densityof the spine. For measurements of other bones, such as the hip, adifferent edge detection routine would be used. Between Steps 8 and 9and between Steps 11 and 12, the operator is given an opportunity tomodify the parameters calculated by the computer and to correct theparameters based on viewing the bone mineral image display. In Step 13the operator decides which vertebral bodies will be included in theregion of interest used to calculate bone mineral content and bonemineral density.

What is claimed is:
 1. A bone densitometer for measuring bone content ina patient comprising an x-ray tube means and associated power supplywhich generates an x-ray beam, means to expose to the x-ray beam a partof a patient that includes regions having bone and adjacent regionshaving only flesh, means to insert into an remove from the x-ray beam apiece of bone-like calibration material of predetermined constantthickness such that said regions of the patient are exposed both to thex-ray beam and to the beam obstructed by said predetermined thickness ofbone-like calibration material, detector means arranged on the oppositeside of the patient to detect x-rays and produce signals correspondingto the amount of x-rays transmitted through the patient, and signalprocessing means responsive to signals from the detector means toproduce a measurement of bone content based upon the signals produced bythe exposure to the x-ray beam and corrected on the basis of signalsproduced by said exposure to the x-ray beam obstructed by saidpredetermined thickness of bone-like calibration material.
 2. The bonedensitometer of claim 1 wherein said signal processing means isresponsive to distinguish, from the set of signals produced by the x-raybeam, a subset of signals which corresponds to regions of the patienthaving only flesh, and said signal processing means being responsive tosignals from said subset as the basis for producing the correction. 3.The bone densitometer of claim 1 wherein the means to expose saidregions to said x-ray beam is a scanning means constructed to scan saidbeam in a pattern over the patient and said means to insert said pieceof bone-like calibration material is constructed and arranged to causeany point of the patient exposed to the unobstructed beam to liesubstantially adjacent to a point exposed to the beam while obstructedby said bone-like calibration material.
 4. The bone densitometer ofclaim 3 in which said means to insert and remove said predeterminedthickness of bone-like calibration material is constructed to act onlines of scan distributed throughout the pattern of the scan.
 5. Thebone densitometer of claim 4 in which said x-ray tube means isconstructed to produce a pencil beam and said beam pattern is a rasterscan pattern.
 6. The bone densitometer of claim 5 in which said means toinsert and remove said piece of bone-like calibration material comprisesa rotating carrier constructed to repeatedly carry said piece ofmaterial into and out of the beam during every line of scan of saidpattern.
 7. The bone densitometer of any of the claims 9, 10 or 1-6wherein said power supply comprising means for applying alternate highand low voltage levels to said x-ray tube.
 8. The bone densitometer ofany of the claims 1-5, wherein the means to inset said piece ofbone-like calibration material into the x-ray beam comprises a rotatingcarrier which carries said piece into and out of the x-ray beam.
 9. Thebone densitometer of claim 1 having a pencil beam collimator arranged toform and direct a pencil beam of x-rays through the patient, saiddetector means aligned with the collimator to detect x-rays attenuatedby the patient, said x-ray tube, pencil beam collimator and detectormeans driven in unison in an X-Y raster scan pattern relative to thepatient, scanning over portions of the patient having bone and adjacentportions having only flesh, and constantly rotating means for causingthe beam to pass repeatedly through said bone-like calibration materialin the course of the scan.
 10. The bone densitometer of claim 9 wheresaid signal processing means is responsive to data based upon x-rayspassing through regions having only flesh to produce the correction. 11.The bone densitometer of claim 9, 10 or 3 wherein the scan pattern islimited to an area within the outer dimensions of the patient.
 12. Thebone densitometer of claim 9 wherein said power supply is constructed toapply alternate high said low voltage levels to said x-ray tube.
 13. Thebone densitometer of claim 12 wherein a control means for controllingthe frequency of alternating said voltage is dependent upon the speed atwhich said x-ray tube, collimator and detector means are driven in scanmotion and the beam width produced by said collimator such that saidcontrol means applies alternating high and low voltage levels to thex-ray tube at a frequency sufficiently high that at least one pair ofhigh and low level exposures occurs during the short time period duringwhich the pencil beam traverses a distance equal to about one beamwidth.
 14. The bone densitometer of claim 13 constructed to producepairs of high and low voltage pulses at a rate of the order of sixty persecond, said x-ray tube, collimator and detector means being drivenalong said scan at a rate of the order of one inch per second and saidcollimator produces a pencil beam of between about one and threemillimeters in diameter.
 15. The bone densitometer of claim 14constructed to cause said power supply to supply alternate high and lowvoltage to said x-ray tube and said x-ray beam passes through saidbone-like calibration material for the duration of every other high andlow voltage pulse pair.
 16. The bone densitometer of claims 9, 10, 12,13, 14 or 1 wherein said detector means comprises an integratingdetector controlled to integrate the detected signal repeatedly overtime periods that are short relative to the time required to advance thex-ray scan pattern by one pixel of resolution.
 17. The bone densitometerof claim 16 including an analog to digital converter to converter eachintegrated value to a digital signal and a digital computer means forproducing said measurement of bone content of the patient by processinga stream of said digital signals.
 18. The bone densitometer of claim 9,10, 13, or 14 including a reference system having at least two referencedetectors, each provided with a different absorber, said referencesystem correcting for both x-ray tube current and voltage changes. 19.The bone densitometer of claim 18 constructed to correct said output ofsaid detector means substantially on the basis of a function of thesignals produced by said reference detectors.
 20. The bone densitometerof claim 19 wherein there are two of said reference detectors and saidfunction is substantially a straight line defined by the detectedsignals of said reference detectors.
 21. The bone densitometer of claim13 wherein said x-ray beam passes through said bone-like calibrationmaterial at least once per scan line for a period during which thepencil beam moves less than about one beam width and wherein saiddetector means comprises an integrating detector controlled to integratethe detected signal repeatedly over short time periods relative to thetime required to advance said x-ray tube in its scan by one pixel ofresolution.
 22. The bone densitometer of claim 13 wherein said x-raybeam passes through said bone-like calibration material at least onceper scan line for a period during which the pencil beam moves less thanabout one beam width and a reference system having at least tworeference detectors each provided with a different absorber, saidreference system correcting for both x-ray tube current and voltagechanges.
 23. The bone densitometer of claim 13 wherein said detectormeans comprises an integrating detector controlled to integrate thedetected signal repeatedly over short time periods relative to the timerequired to advance said x-ray tube in its scan by one pixel ofresolution and a reference system having at least two referencedetectors each provided with a different absorber, said reference systemconstructed to correct for both x-ray tube current and voltage changes.24. The bone densitometer of claim 13 wherein said x-ray beam passesthrough said bone-like calibration material at least once per scan linefor a period during which the pencil beam moves less than about one beamwidth, said detector means comprises an integrating detector controlledto integrate the detected signal repeatedly over short time periodsrelative to the time required to advance said x-ray tube in its scan byone pixel of resolution, and a reference system having at least tworeference detectors each provided with a different absorber, saidreference system constructed to correct for both x-ray tube current andvoltage changes.
 25. The bone densitometer of claim 9, 10, 12, 13 or 14in which the bone-like calibration material is rotated on a disc thatcauses a pencil beam to periodically pass through the calibrationmaterial for a period sufficiently small that the distance traversed bythe pencil beam during the interruption is not more than about one beamwidth.
 26. The bone densitometer of claim 25 wherein said detector meanscomprises an integrating detector controlled to integrate the detectedsignal repeatedly over short time periods relative to the time requiredto advance said x-ray tube in its scan by one pixel of resolution. 27.The bone densitometer of claim 25 including a reference system having atleast two reference detectors each provided with a different absorber,said reference system constructed to correct for both x-ray tube currentand voltage changes.
 28. The bone densitometer of claim 25 wherein saiddetector means comprises an integrating detector controlled to integratethe detected signal repeatedly over short time periods relative to thetime required to advance said x-ray tube in its scan by one pixel ofresolution, a reference system having at least two reference detectorseach provided with a different absorber, said reference systemconstructed to correct for both x-ray tube current and voltage changes.29. A method of measuring bone content in a patient who is held in fixedposition comprising:generating an x-ray beam, scanning the patient bypassing the x-ray beam through portions of the patient having bone andadjacent portions having only flesh, passing the x-ray beam through abone-like calibration material having a predetermined constant thicknessin the course of scanning the patient such that portions of the patientare exposed both to an unobstructed x-ray beam and to a beam obstructedby said thickness of bone-like calibration material, detecting x-raysattenuated by the patient, calibrating signals from detected x-raysusing data based upon x-rays attenuated by said calibration material inregions having only flesh, and processing signals from detected x-raysto provide a measurement of bone content in the patient.
 30. The methodof claim 29 comprising the further step of driving said x-ray beam in anX-Y raster scan pattern relative to the patient.
 31. The method of claim30 further comprising periodically passing said x-ray beam through saidbone-like calibration material a multiple number of times per scan line.32. The method of claim 29 or 30 further comprising passing said x-raybeam through said bone-like calibration material at least once per scanline.
 33. The method of claim 29 or 30 wherein the signal calibratingcomprises selecting data based upon x-rays attenuated by saidcalibration material in regions adjacent to bone.
 34. The method ofclaim 29 or 30 further comprising restraining said x-ray tube fromscanning beyond the outer dimensions of the patient.
 35. The method ofclaim 29 or 30 further comprising applying alternating high and lowvoltage levels to an x-ray generation means to generate said x-ray beam.36. The method of claim 29 or 30 further comprising:attenuating portionsof said x-ray beam with at least two different reference absorbers,detecting x-rays attenuated by said reference absorbers, and correctingfor variations in said x-ray beam using signals detected from x-raysattenutated by said reference absorbers.